1. Field of the Invention
The present invention relates to a process for producing an information recording medium. More particularly, it relates to a process for producing an information recording medium comprising joining a disc-shaped resin film to a disc-shaped resin substrate.
2. Description of Prior Art
Information recording medium using high energy density beam such as laser beam has been developed and put to practical use in recent years. The information recording medium is called optical disc and can be used as video disc, audio disc and disc memory for large-capacity static image files and large-capacity computers.
The optical disc basically comprises a recording layer provided on a disc-shaped transparent substrate of a plastic material or glass. An undercoat lay or intermediate layer of a high-molecular material can be provided on the surface (on which the recording layer is provided) of the substrate from the viewpoints of improving the smootheness of the surface, adhesion between the substrate and the recordingw layer and the sensitivity of the optical disc.
The recording layer is susceptible to external influences so that it must be protected from the external influences. Thus, there are generally used optical discs having an air sandwich structure where a substrate having a recording layer thereon is joined to another substrate which may have a recording layer via inner and outer spacers in such a state that the recording layer is positioned inwards.
However, the optical disc having an air sandwich structure has disadvantages in that the manufacturing process thereof is complicated and as a result, the manufacturing cost thereof is high and in that the thickness of the disc is large and hence it may be hard to handle.
In order to solve the above-described problems, there had been proposed a method in which a disc-shaped resin film is joined to a surface of the substrate, on said surface side of substrate being provided the recording layer (see, Japanese Patent Provisional Publication No. 51(1976)-75523). In this method, the film is joined to the disc substrate by means of an adhesive in the course of assembly of the optical disc. However, when the bonding of each member is carried out by means of an adhesive, it takes from tens of seconds to several minutes to coat and cure the adhesive per disc, so that there is much difficulty in mass-producing the disc. Further, there is possibility that the adhesive is squeezed out on the outer and inner peripheries of the substrate or on the side, on which the recording layer is provided , when the film is joined to the substrate. When the adhesive is squeezed out on the recording layer side of the substrate, the adhesive sticks to the recording layer and as a result, the recording layer deteriorate. Further, there is a problem that the sheet-form film is peeled off from the substrate by environmental change or with the passage of time, when they are joined to each other by means of an adhesive.
Japanese Patent Provisional Publication No. 16(1986) -104375 proposes a joining method wherein the sheet-form film is joined to the substrate by means of sewing, baking, adhesive tape, pinning and nailing in addition to the use of the adhesive.
However, this method causes the following problems. For example, the baking has a problem with regard to the accuracy of the substrate, since heat or load is applied to the disc substrate and optical properties are adversely affected. Further, there is possibility that recording characteristics of the recording layer near the joined part are deteriorated by heat. The joining by pinning or nailing has problems in that the manufacturing process is complicated and there is possibility that the film is broken during handling at a position where the film is fixed, since the film is thin. The joining by sewing or adhesive tap has also disadvantages in that the manufacturing process is complicated, thread or tape is aged with the passage of time and the joined deteriorates or the appearance is spoiled.
With regard to the joining of the substrates to each other, there had been proposed a method wherein ultrasonic wave is applied (see, Japanese Patent Provisional Publication No. 58(1983)-14342). In this method, ultrasonic wave is used to join the substrates to each other. However, since ultrasonic wave is applied to both surfaces of the substrates, large energy is required for satisfactorily welding them. Thus, there is a problem that the substrates are badly affected and this method is practically not advantageous.